To use the Out of Office Assistant, you must be using
Outlook and an Exchange server. If you are not on an
Exchange server, then you would need to use rules to reply
to messages.
However, rules will not allow you to distinguish who to
send an Out of Office reply to so all spam that you
recieve will have the reply sent, letting the spammers
know that they have a "live" address.
This only works when you're connected to a Microsoft Exchange Server. If
not, you'll have to use hte Rules Wizard and leave Outlook running and
connected to the Internet. Even then it's not the greatest of ideas as it
will reply to spammers as well and confirm you have a valid email address.